In recent years, for the purpose of solving global environmental issues and energy issues, introducing hybrid electric vehicles (HEV) and electric vehicles (EV) and expanding the market share thereof has been drawing attention as effective means for saving oil consumption to about a half or less. In particular, HEV having already been put into practical use, presently, with nickel-hydrogen batteries (Ni/MH batteries) having improved power characteristics mounted, is expanding its market rapidly. Further, with a view to mounting to the HEV with expanding market, not only improving the characteristics of Ni/MH batteries but also motivation of developing lithium-ion batteries, improved capacitors and the like are vigorous.
Since this has been used under harsh conditions in which many batteries are connected in series and exposed to broadly ranged temperature, vibration or the like, much more improved reliability than the conventional HEV has been required, and further, at the same time, powering up a power source (improvement in high-rate discharge property) has been also required for the purpose of starting and running HEV and the like.
The present application suggests with a view to improving energy density (small size: Wh/L and light weight: Wh/kg) and reducing the cost of electrochemical applied products represented by secondary batteries such as Ni/MH batteries and lithium-ion batteries and capacitors, and to be specific, it provides a means to achieve a goal by improving electrode substrates of electrodes to be used.
In addition, although the present invention is extremely effective for particularly electrochemical applied products for motive power use such as HEV, it is also applicable to generally used electrochemical applied products.
Hereinafter, detailed explanation will be made with respect to a cylindrical sealed Ni/MH battery, which has already been put into practical use as a power source for HEV. Here, this Ni/MH battery refers to an 1.2 V alkaline secondary battery in which a nickel electrode using nickel oxide powder is used as a positive electrode, a negative electrode using hydrogen absorbing alloy powder is used as a negative electrode, a porous non-woven cloth of synthetic fiber is used as a separator, and alkaline solution comprising potassium hydroxide (KOH) as a major component is used as electrolyte.
In developing the Ni/MH battery developed and put into practical use around 1990s in HEV use required for power characteristics, the battery basically employs the same structure as that of the existing nickel-cadmium batteries (Ni/Cd batteries). In other words, the battery employs the following structure. An electrode group with a positive electrode and a negative electrode spirally-rolled interposing a separator therebetween, which is thinner than generally used batteries, is contained in a bottomed cylindrical can. In the positive electrode, a part of a holder (i.e., an electrode substrate) of active material powder (mainly, nickel hydroxide, Ni(OH)2) is electrically connected to a cover body interposing a metal-made current collecting plate and an electrode lead plate therebetween. In the negative electrode, a nickel-plated punched steel (NPPS) substrate is contacted with a bottom portion of the bottomed cylindrical can or partly welded, thereby being electrically connected to the bottomed cylindrical can.
In the case where the size of the battery becomes larger as in the battery for HEV use, when paste type electrodes having high energy-density and light weight are employed in the cylindrical battery, a possibility of micro short circuit is increased by falling-off of the active material powder from cut edge portions of both the electrodes. For this reason, there has been used a sintered type positive electrode with very little falling-off of the active material powder from the cut edge portions of the electrodes, that is, a battery which inhibits this issue.
However, the cylindrical sealed Ni/MH battery employing the sintered type positive electrode contains a large amount of nickel, its energy density is small, and nitrate is mixed into the battery during the manufacturing process. As a result, the battery gets heavy and large, and it has a self-discharge problem. For this reason, it has been important issue to make full use of the paste type positive electrode in the cylindrical sealed Ni/MH battery.
Therefore, the present inventors solved the problems of micro short circuit by first using a flexible three-dimensional substrate (3DF, 3-dimensional foil) as the substrates of both electrodes (Patent Literature 1), and bending the electrode edge that corresponds to an electrode lead portion together with a separator to form a bag of the separator, and they put the paste type electrodes into practical use (Patent Literature 2). As a result, the cylindrical sealed Ni/MH battery which is made small in size, light in weight and low-cost has been able to be provided. However, there are still some issues in mass manufacturing of the three-dimensional substrate to be used and in rollers to be used for processing of the electrodes.